They are of two grammatical categories, the first is a noun and the second an adverb. It is not a matter of nuance, that is semantical nuance, but a matter of alternative formulation; here is the idea;

There is no difference in meaning. In the firs case whoever is speaking chooses to specify the time by using a noun telling that it is a time near a given point in time (to be made precise, otherwise it's all meaningless, incomplete); in the second they choose instead the precise point in time and specify that it has been chosen approximately, which amounts to the same thing.

I The first is used to specify a place or a time that is found in the vicinity of a precise point in time or a precise location, respectively. You can translate it for instance by "vicinity" or "neighbourhood" or "whereabouts" when place is the concept; when time is the concept you resort usually to an adverb in English, the typical ones being "about", "approximately" and "near", although you can also use the word "time" modified by an adverb ;

II The second, as the grammatical category shows, is used to modify a noun; however it must mostly be an expression for a measure, often used with a determiner ("environ" is then a predeterminer); some nouns can be modified, those are nouns that refer to points in time ( midi, minuit, Pâques, …).
It cannot be used to modify nouns of location except if specified by a measure;

A couple of points are worth mentioning relative to the nouns refering to points in time ; the adverb can be postponed and sometimes has to be postponed ; in some cases it is preferable not to use the adverb.

Ils seront prêt à environ midi. or Il seront prêt à midi(,) environ.

Sa tante lui rend visite aux environs de Noël. (à environ Noël; the why of this usage is not clear.)

I think I kind of have a handle on how it works. I’m confused with what you mean by “nouns of measures,” I’m sure it speaks for itself, but if you could clarify a bit more, that would be great. “Measures” seems to be literal in that you’re measuring something, given your examples. But then you used it to describe the bad idea of speeding around the capital, so there’s where I’m confused.
– tssmith2425May 18 at 23:22

Also, is “vers environ” redundant? Or does it mean something like “around about...”?
– tssmith2425May 18 at 23:22

@tssmith2425 It seems very few people will say that, it is redundant and I wouldn't use it; however it appears that quite a number of people do use that; I find it can be translated as "about approximately" which is no doubt "around about" ; I'm not familiar with this adverbial locution but I just found out it means "à peu près" and that is "environ" (Soyez à boutique à à peu près huit heures. or Soyez à la boutique à environ huit heures.). You're right about this absurdity relative to measures and nouns; sorry for that; I'll try to make it right, now.
– LPHMay 19 at 5:44

Thank you for further explaining. I still find it confusing, but if the meanings are identical, then perhaps I will have to commit to memory the individual uses over time.
– tssmith2425May 19 at 13:59

What is the meaning of “pour” in your final three bullet points? I translate those phrases as saying “there’s some until about the summer,” but I don’t understand the role of “pour” here.
– tssmith2425May 19 at 14:02